1. Field of the Invention
This invention relates to test apparatus, and more particularly concerns an improved edge strength tester having an indenting member and means for controlling the velocity and force exerted upon said indenting member.
2. Description of the Prior Art
Cutting implements are in general subject to utilization by virtually every household and industry for the cutting or severing of an indeterminable number of items. In each environ, whether domestic, industrial, or the like, quite obviously the efficiency of the blade or cutting implement is dependent upon a properly sharpened cutting edge.
Various techniques are utilized for creating sharpened cutting edges. Often it is assumed that because an edge has been subjected to a sharpening process, that the proper sharpness has been achieved. However, incorrectly sharpened edges may result from grinding wheel burning, wire edges or burrs, large tip radii, improper hardening, incorrect geometry, and other factors.
In the industrial production of cutting blades, much attention must be given to the particular sharpening technique utilized for each type of blade, and its intended usage. The ability to measure edge strength is accordingly necessary for purposes of optimizing and monitoring production techniques.
Edge strength, a criterion of effective performance characteristics, is related to the blade edge angle and the hardness of the edge material.
Various concepts have been employed in attempts to determine the sharpness of a cutting edge. Such edges may include but are not limited to knife blades, razor blades, surgical scalpels, industrial slitters, chippers, granuletots, etc. For example, U.S. Pat. No. 4,528,843 to Juranitch discloses an edge sharpness tester which includes an elongated body of material into which a properly sharpened edge only will bite when slid lightly therealong at a predetermined angle. However, there is no correlation between test results and strength of the edge or its effectiveness in use.
U.S. Pat. No. 4,178,797 to Kozlowski, Jr, provides a machine for testing the relative sharpness of surgical knife blades wherein the blade is forced into contact with a rotated rod. The blade cuts into the rod more deeply with each turn. The number of turns required to sever the rod determines the relative sharpness of an edge. The Kozlowski tester is not capable of measuring small differences in extremely sharp edges due to its inability to control the velocity at which the test blade approaches the rod, the resultant force exerted by the blade upon the rod, and the frictional force of the rod on the opposite facets of the edge, The Kozlowski tester does not ascertain whether the dullness is a result of a weak edge geometry which crumbles during the test or an initially dull but strong edge. The measured frictional force may not be related to the final use of the edge.
U.S. Pat. No. 3,931,732 to Yon Heirlinger discloses a hand held tool for testing the edge sharpness of non-cutting edges for the purpose of hazard prevention. The device employs a rotatable mandrel carrying a covering of testing material which contacts the edge to be tested.
Various non-contacting test instruments have been disclosed in recent years. For example, U.S. Pat No. 5,196,800 to Graff et al. discloses a non-contacting sharpness tester adapted to measure the sharpness of the edge of a circular slitting knife. This technique involves repeatably placing a capacitance sensor probe symmetrically over the edge at a predetermined distance from a calibrated sharp edge and measuring capacitance to derive a measurement that varies with knife wear. This measures wear from use, not initial sharpness. The Graff device is insensitive to the small tip radius changes of razor blade edges, and gives no indication of the initial condition of the edge prior to use. Accordingly, it cannot be employed as a process correction device.
A testing device which had widespread early usage in industry is based upon the principle of optically measuring the penetration of a restrained blade edge into a standard material under a fixed load, This method indented the edge with a light load of a few grams, pushing a diamond wedge with an included angle of 170 degrees. The large included wedge angle made minimum penetration into the edge so that very small depths could be sampled. The large angle made an elongated indent along the edge which could be measured by a filar eyepiece on a high magnification microscope. This is an expensive and time consuming method for controlling production.
In more recent industrial usage, the depth of the indent is measured directly. A minor load (about one gram or less) is used to locate the edge, and a heavier load (30 grams) deforms the bulk of the edge. If a displacement gauge is placed with the diamond wedge, the difference of penetration can be measured directly. It has been found that this reading gives a direct indication of the blade material hardness, the geometrical angle of the edge and the tip radius. It also correlates well with the optical method.
One of the difficulties of this method is that building vibration, namely seismic effects on the building, cause the readings to increase in an uncontrolled manner, giving an erroneous indication of penetration. This vibration is related to the inertia of the mechanical system holding the diamond wedge. It is particularly important to keep the mass of the members of the instrument to a minimum. These members are the diamond Indenter, the displacement gauge, the force or load-imparting device and the mechanical parts holding them together. In particular, this mass should be less than 50 grams.
Further, the velocity of the diamond wedge approaching the edge must be controlled so that the momentum of the diamond does not transfer sufficient energy so that it exceeds the force of the minimum load. This energy is a function of both the mass of the assembly and its velocity, and both must be minimized to get true readings. It is the purpose of this invention to keep the mass small as well as to incorporate a velocity control which prevents excessive speed.
It is therefore an object of the present invention to provide an edge strength test instrument capable of producing precise, accurate, and repeatable measurements.
It is another object of the present invention to provide a tester of the aforesaid nature having means for controlling the approach velocity of a penetrator element to the blade.
It is yet another object of this invention to provide a tester of the aforesaid nature wherein the mass of the penetrator element and associated components is minimized.
It is a further object of this invention to provide a tester of the aforesaid nature which is adaptable to various blades, quick in operation and capable of providing information useful in remediating the blade-producing process.
These and other beneficial objects and advantages will be apparent from the following description.